A passive optical network (PON) delivers voice, video and other data among multiple network nodes using a common optical fiber link. Passive optical splitters and combiners enable a number of network nodes to share the optical fiber link. Each network node terminates the optical fiber link for a residential or business subscriber, and is sometimes referred to as a subscriber premises node. A PON typically includes a PON interface having multiple, independent PON interface modules serving multiple optical fiber links In particular, each PON interface module provides an interface for transmission and reception of data over a particular optical fiber link that serves a group of network nodes.
Typically, both the PON interface and each of the network nodes include one or more segmentation and reassembly (SAR) engines. A SAR engine within the PON interface receives units of data, such as Internet packets, telephone voice data, cable television data and the like, segments the units of data into smaller subunits, e.g., ATM Adaptation Layer-5 (AAL-5) cells, and bounds the subunits into bounded units, such as frames, to preserve the data. A SAR engine within a network node reassembles the subunits into the original units of data once the last of the bounded subunits arrives, thereby reconstructing the original flow of data for delivery to devices served by the network node.
Generally, a SAR engine is required for each microflow. A microflow refers a flow of data that can be uniquely identified by lower level identifiers, e.g., ATM Virtual Path Identifier (VPI)/Virtual Channel Identifier (VCI) pairs. For example, a microflow can carry data for a single cable television channel, a telephone exchange, an Internet connection, or the like. A PON interface typically includes multiple SAR engines to concurrently provide data from multiple microflows. A network node may similarly include multiple SAR engines to handle each of several concurrently requested microflows. However, the need for multiple SAR engines increases the complexity and cost associated with each network node.
Various techniques have been developed to address the high complexity and cost associated with PON networks. For example, some PON interfaces and network nodes have been designed to implement merging techniques, such as virtual channel (VC)-merge. VC-merge is a technique that involves merging the units of information from multiple microflows into a common flow. According to the VC-merge technique, a SAR engine in a PON interface interleaves units of information having different source microflows. In particular, a SAR engine implementing VC-merge may interleave subunits from different source microflows within a common bounded unit. In the AAL-5 example, the subunits are cells and the bounded unit is a frame. Because the microflows are merged into a common flow, the network nodes only require a single SAR engine to receive all of the microflows.
Although VC-merge techniques may reduce the complexity of the network nodes, using a common flow generally prevents the use of conventional traffic metering mechanisms that rely on lower level identifiers, such as ATM traffic metering mechanisms. Also, with a common flow, the units of information share a common VPI/VCI identifier. Therefore, in order to select desired units of information, network nodes must rely on higher level identifiers to identify the microflows and reassemble the corresponding original units of information. However, various units of information, such as MPEG-2 packets, may not provide higher level identifiers.
In these instances, a single SAR engine may be unable to resolve the source of a unit of information and thus cannot distinguish between different microflows for purposes of accepting or rejecting individual units of information. In the event the network node is configured to accept all unknown units of information, the network node can quickly overflow with information, which in turn leads to network inefficiencies. On the other hand, if the network node is configured to reject all unknown units of information, these and other types of information lacking higher level identifiers will be unavailable to subscribers.